Conservation of Momentum

A railroad flatcar of weight 7840 N can roll without friction along a straight horizontal track. Initially, a man of weight 596 N is standing on the car, which is moving to the right with speed 25 m/s; see the figure. What is the change in velocity of the car if the man runs to the left so that his speed relative to the car is 8.0 m/s?

I set up the equation.
[math ]m_1*v_1_i+m_2*v_2_i=m_1*v_1_f+m_2*v_2_f[/tex]
[math ]0+0=\frac{596}{9.8}*8+\frac{7840}{9.8}*v_2_f[/tex]